1. Field of the Invention
The present invention relates to a styrene resin composition comprising a linear polystyrene and a multibranched polystyrene, and a process for producing such a styrene resin composition. Compared to a linear polystyrene of the same weight average molecular weight, a styrene resin composition of the present invention has a higher melt mass-flow rate and a superior fluidity during molding, and is consequently ideally suited for use as a molding material in a variety of molding processes.
2. Description of Related Art
Because styrene based resins exhibit good rigidity and excellent dimensional stability, and are also cheap to produce, they are widely used in molding applications, in recent years, improved strength has been sought from styrene based resins in a number of applications. Although increasing the molecular weight of a styrene based resin is an effective method of increasing the strength of the styrene based resin, in the case of conventional linear polystyrenes, increasing the molecular weight invites an increase in the melt viscosity, with a resulting marked deterioration in the moldability.
The addition of plasticizers is typically used as a method of improving this type of deterioration in moldability, however this causes a reduction in the mechanical strength of the final resin molded product. One example of a process which aims to resolve this problem by imparting a branched structure to the polystyrene is disclosed in Japanese Examined Patent Application, Second Publication No. Sho 41-19511, wherein branching is introduced into the polystyrene by using styrene and a polyfunctional peroxide compound such as 2,2-bis(4,4-di-t-butyl-peroxy-cyclohexyl)-propane as a polymerization initiator, although even with this process, the degree of branching can only be controlled at a low level, and achieving a satisfactory branched structure is difficult.
Japanese Unexamined Patent Application, First Publication No. Hei 7-166013, in an attempt to impart a branched structure to polystyrene and give a styrene based resin with a high molecular weight and a favorable level of moldability, discloses a polystyrene based copolymer with a weight average molecular weight of 200,000 to 2,000,000 produced by introducing branched structures into its polystyrene chain by copolymerizing styrene with a compound such as divinylbenzene containing at least two vinyl groups. The formed polystyrene based copolymer is disclosed as exhibiting excellent moldability, although because the copolymer is prone to gelation during the polymerization process, it is difficult to produce industrially, and because the amount of the other compound having at least vinyl groups is restricted, achieving significant increases in the physical properties of the product resin is difficult.
Japanese Unexamined Patent Application, First Publication No. Hei 9-316261, in another attempt to give a styrene based resin with a high molecular weight and a favorable level of moldability, discloses a mixture of a star branched polystyrene produced by an anionic polymerization, and a linear polystyrene produced by a radical polymerization, and reports that a styrene based resin composition with good fluidity and mechanical strength, and favorable moldability can be produced with no loss of rigidity or heat resistance. However, because an anionic polymerization is used, a complicated control is required, for making the process unsuited to industrial production.